A slide presentation accompanying Duterte's request, seen by AFP, compared the Marawi crisis to the Islamic State takeover of the Iraqi city of Mosul.

Marawi itself could now become a magnet for foreign fighters in Iraq and Syria, it said.

Most of the militants' leaders remain at large, the presentation added, while about 90 of the gunmen have slipped past security cordons and can link up with other armed groups in the region to mount similar widescale attacks.

At the hearing, defence and security officials justified the need for martial law, saying that aside from Marawi, Islamist militants were planning attacks in other parts of the southern Philippines.

They said almost a thousand pro-IS militants, holding 23 hostages, were still active elsewhere in the south.

In Marawi, the military said only about 60 gunmen were left in a 49-hectare (121-acre) area of Marawi, but Duterte said he needed martial law powers to rebuild the city and ensure the war did not spread elsewhere.

"I cannot afford to be complacent," Duterte told reporters Friday, adding the military would be conducting further "mopping up operations" even after they recapture Marawi.

"If there is a spillage it will not be as bad if you have this stopgap," he added.

Duterte imposed 60-day martial rule -- the maximum period allowed by the constitution -- over the Mindanao region on May 23 within hours of the gunmen beginning their rampage.

On Monday he asked Congress to extend it until the end of the year, along with the continued suspension of a constitutional safeguard against warrantless arrests.

In an unprecedented move, both the House and the Senate met jointly on a weekend to vote on Duterte's request.